Method for manufacture of cast fluoropolymer-containing films at high productivity

ABSTRACT

A method is provided for preparation of a fluoropolymeric film on a carrier, comprising: 
     (a) preparing an aqueous dispersion comprising a fluoropolymer; 
     (b) dipping a carrier belt through the dispersion such that a coating of the dispersion is formed on the carrier belt; 
     (c) passing the coated carrier belt through a metering zone to remove excess dispersion; 
     (d) drying the metered coated carrier to remove the water from the dispersion; and 
     (e) heating the dried coated carrier to a temperature sufficient to consolidate the dispersion, wherein the carrier belt is formed from a material of low thermal mass having chemical and dimensional stability at the consolidation temperature of the dispersion and a work of adhesion between the carrier belt and the dispersion that does not exceed the yield strength of the consolidated fluoropolymeric film.

BACKGROUND OF THE INVENTION

The present invention relates to a method of producing fluoropolymerfilms at high productivity and to films produced using the method.

The production of thin plastic films has generally been accomplishedusing one or more of three processes: melt extrusion, casting fromsolutions or organosols, and casting from aqueous dispersions. Meltextrusion of films is generally preferred to casting since it does notrequire the removal of an organic solvent, water, or surfactants. Ittherefore produces a very clean film and typically is characterized byhigh productivity. Melt extrusion cannot, however, be used for allmaterials.

Casting methods are preferred if the required time at extrusiontemperature is sufficient to result in thermal or oxidative degradationof the polymer. Casting is also preferred when the melt viscosity of thepolymer is sufficiently high to make extrusion either technicallyimpossible or economically impractical.

In the case of fluoroplastics, all three processes are used to producefilms, with the choice of process largely depending on the monomercontent of the polymer. The most common monomers presently employed toproduce fluoroplastics include tetrafluoroethylene (TFE),chlorotrifluoroethylene (CTFE), vinylidene fluoride (VF₂), and vinylfluoride (VF). All of these are available as homopolymers; i.e., PTFE(e.g. "Teflon"), PCTFE (e.g. "KelF"), PVF₂ (e.g. "Kynar"), and PVF (e.g."Tedlar"), respectively. PCTFE and PVF₂ are melt extrudeable as thinfilms with some difficulty due to the fact that the time/temperaturehistory during extrusion is near to that which could result in polymerdegradation at the severe shear rate of melt extrusion. This conditioncan be further aggravated in the presence of certain fillers. PVF filmcannot be produced by melt extrusion due to thermal instability and thusis produced by a casting process and subsequently is biaxiallystretched. Homopolymer PTFE cannot be practically melt extruded at alldue its extraordinarily high melt viscosity.

In order to overcome such problems in the case of melt extrusion ofthese homopolymers, copolymers of these monomers have been developedwhich are generally lower in melting temperature and melt viscosity atextrusion temperatures. This allows extrusion of the polymers attemperatures at which no significant thermal degradation occurs.Consequently, fluoropolymer films are most generally based upon suchreadily extrudeable copolymers. These include copolymers of TFE withhexafluoropropylene, e.g., "Teflon" FEP, or with perfluoroalkyl vinylethers, e.g., "Teflon" PFA, or with ethylene, e.g. "Tefzel" ETFE.Similarly, copolymers of CTFE include those with vinylidene fluoride orhexafluoropropylene, e.g., "Kynar", as well as with ethylene, e.g.,"Halar". Terpolymers of these basic monomers are also known and used inextrusion.

Since pure PTFE cannot be melt extruded, as mentioned above, otherprocesses have been developed for film production. One such methodinvolves the skiving of thin film from a molded and sintered billet.Another involves the casting of an aqueous dispersion onto a metalliccarrier. The deposited resin is subsequently stripped from the carrierto yield a very high quality film relative to the skived films.

A casting process for PTFE is described in U.S. Pat. No. 2,852,811issued to John V. Petriello in 1958, which is incorporated herein byreference. In summary, this process involves continuously depositing alayer of a PTFE dispersion onto a metal carrier, drying the coatedcarrier and then sintering the dried coating. These steps are thenrepeated until a film of the desired thickness had been formed. The filmis then stripped from the carrier. U.S. Pat. No. 2,852,811 stresses theimportance of the nature of the carrier belt used in the castingprocess. Thus, highly polished, corrosion resistant metal carrier beltshave been used in subsequent casting efforts.

Cast PTFE films exhibit virtually no mechanical anisotropy, and havesubstantially higher tensile strength, elongation, and dielectricbreakdown strength than skived PTFE films. Unfavorable processeconomics, however, have prevented a wide acceptance of casting as amethod for making fluoropolymer films. Among the factors affecting theeconomics are the properties of the metal carrier belts. These belts arefairly rigid and heavy, and thus require a special tracking mechanism todrive the belt through the apparatus. This essentially fixes the widthof the material produced, causing a loss in versatility.

The casting process as described by Petriello also suffers as a resultof low productivity. In an effort to elaborate upon the significantprocess parameters affecting film quality, investigations were sponsoredby the Aeronautical Systems Division of the United States Air Forcebetween 1955 and 1962 which resulted in the publication of a reportentitled "Production Refinement of Very Thin Teflon Film." Thispublication emphasized the importance of the dispersion characteristicsand line speed as each can significantly affect the quality of the castfilm. Specifically, the Air Force study observed that the quality offilm produced by the casting method deteriorates very rapidly at linespeeds above 3 feet per minute. (See P. 19 "Production of Very ThinTeflon Film"). Productivity of film manufacture at such slow rates is ingeneral prohibitively costly: even simple, monolithic cast films of PTFEmust be sold at four to five times the price of skived PTFE or two tothree times that of extruded FEP to be economically attractive. This hasled to very minimal acceptance of cast films in the marketplace and hasbeen the major cause of the lack of continued research over the pastdecades into processes for casting fluoropolymer films.

Additionally, the very low critical cracking thickness of mostfluoropolymer dispersions suggest that thicker individual lamellaewithin any given film cannot be achieved to even partially offset thepoor productivity associated with very low linear line speed.

It is of interest to note, however, that all of the previously mentionedfluoroplastic homopolymers and copolymers are available as aqueousdispersions and can be used to produce cast films. Moreover, the castingprocess potentially offers several distinct advantages over theextrusion process for producing films. The casting process inherently isa multi-layering process; thus, multi-layer film production by castingmethods avoids the intrinsic problems and substantial unit investmentwhich would be associated with coextrusion or extrusion coating offluoropolymers. PTFE films with surface(s) of fluorinated ethylenepropylene (FEP) or perfluoroalkoxy resins (PFA) are availablecommercially from casting equipment. Additionally, the casting ofalloyed fluoropolymers, including both thermoplastic and elastomericpolymers and which may optionally incorporate metal, mineral, or ceramicadditives to modulate chemical, optical, electrical, and magnetictransport properties of film is facilitated by the casting process inboth monolithic (uniform composition) and complex (non-uniformcomposition) film format. Such films are described in commonly assignedU.S. patent application Ser. Nos. 600,002 and 908,938, and U.S. Pat.Nos. 4,555,543 and 4,610,918, all four of which are incorporated hereinby reference. Most importantly, such a process permits one to combine ina single layer, or in sequential layers, polymers with widely differentmelting temperatures and degradation temperatures since thetime/temperature history of the film as it is processed can be kept muchshorter than that characteristic of melt extrusion.

In short, the casting process is an inherently much more powerful methodthan the extrusion process for producing high quality films with a farlarger number of compositional degrees of freedom. It is an object ofthe present invention to provide a method for the production offluoropolymer films in which the relationship between productivity andfilm quality is dramatically altered such that one can economically takeadvantage of this superiority. The products of this process could enjoysignificant use in electrical and electronic applications as well as inselective membranes and other chemical applications.

SUMMARY OF THE INVENTION

This object is achieved in accordance with the invention using a methodfor preparation of a fluoropolymeric film on a carrier, comprising:

(a) preparing an aqueous dispersion comprising a fluoropolymer;

(b) dipping a carrier belt through the dispersion such that a coating ofthe dispersion is formed on the carrier belt;

(c) passing the coated carrier belt through a metering zone to removeexcess dispersion;

(d) drying the metered coated carrier to remove the water from thedispersion; and

(e) heating the dried coated carrier to a temperature sufficient toconsolidate the dispersion, wherein the carrier belt is formed from amaterial of low thermal mass having chemical and dimensional stabilityat the consolidiation temperature of the dispersion and a work ofadhesion between the carrier belt and the dispersion that does notexceed the yield strength of the consolidated fluoropolymeric film.

Thus, the method of the present invention defines a film casting processwhich differs from that of the prior art in at least two criticalaspects--the use of metering equipment to define the amount offluoropolymer dispersion on the carrier belt and the use of a carrierbelt having a low thermal mass. These alterations allow operation of thesystem at line speeds in excess of 10 linear feet per minute, a speedthat substantially improves the productivity relative to the prior art.At the same time, the films made in accordance with the presentinvention suffer little, if any, reduction in the quality of filmsproduced at higher productivity relative to the prior art.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 shows a schematic of an apparatus for carrying out the method ofthe invention.

DETAILED DESCRIPTION OF THE INVENTION

In accordance with the invention, fluoropolymer films are formed bycasting onto a carrier belt having low thermal mass. This carrier beltis preferably part of a casting apparatus such as that depicted inFIG. 1. The carrier belt 1 is dipped through a fluoropolymer dispersion2 in a dip pan 3 at the base of a casting tower 100 such that a coatingof dispersion 2' forms on the carrier belt 1. The coated carrier belt 1then passes through a metering zone 4 in which metering bars 5 removeexcess dispersion from the coated carrier belt. After the metering zone,the coated carrier belt passes into a drying zone 6 which is maintainedat a temperature sufficient to remove the carrier liquid from thedispersion giving rise to a dried film. The carrier belt with the driedfilm then passes to a bake/fuse zone 7 in which the temperature issufficient to consolidate or fuse the fluoropolymer in the dispersion.Finally, the carrier belt passes through a cooling plenum 8 from whichit can be directed either to a subsequent dip pan to begin formation ofa further layer of the film or to a stripping apparatus such as thatillustrated in U.S. Pat. No. 2,852,811.

In a preferred embodiment of the invention, the bake/fuse zone 7 isheated using dual heat sources, a conventional oven 9 that maintains thetemperature at about 300° F. to 710° F., and a radiant electric heater10 that raises web the temperature to one sufficient to consolidate thefluoropolymer, i.e., about 700° F. or higher in the case of PTFE.

The method of the invention provides superior performance by varyingseveral aspects of the previously known method for casting fluoropolymerfilms: (1) the nature of the carrier or casting medium, (2) the natureof the casting fluids, (3) the nature of the metering methods used toapply the casting fluids to the carrier, and (4) the state ofconsolidation of the polymers as they proceed from the drying fluid tothe fused and recrystallized or solidified films. Each of these requiressome discussion to understand the significant advances of the presentinvention as distinguished from the prior art.

1. Nature of the Carrier

The state-of-the-art teaches the suitability of metallic carriers whichare preferably made of stainless steel polished to a specific surfacesmoothness to maintain sufficient adhesion to hold the in-process filmto the carrier, but not so rough as to provide an anchorage which couldprevent stripping entirely or lead to distortion of the film duringstripping. Aluminum foils are also useful as a carrier, but are lesssatisfactory than stainless steel for several reasons: they are quicklyannealed during high temperature fusing of the applied polymers and,therefore, readily damaged in subsequent use. They are also prone tocreasing or wrinkling in-process and susceptible to chemicalmodification of their surfaces by the aqueous ammoniacal solutionscharacteristic of many fluoropolymer dispersions.

The primary disadvantage of the preferred stainless steel carrier of theprior art is its need to be fully tempered and relatively stiff fortracking purposes. This results in a carrier with sufficient storedmechanical energy under tension to be difficult to manage at high speed.Thus, while suitable at line speeds of 3 to 8 fpm, it is not a goodcandidate for higher speed (10 to 30 fpm) operation. Additionally, theactual adhesion of consolidated fluoropolymer films to the steel is afunction of the number of uses of the steel belt which needs periodicrefurbishment to reestablish appropriate adhesion for strippability.Lastly, at the 5 to 8 mil gauge employed for trackability/mechanicalstability, the steel belt possesses substantial thermal mass relative tothe much thinner (0.15 to 1.5 mil) depositions of resin in any onelamellae. This leads to repeated heating and cooling requirements in thecasting process which is wasteful of energy and limits the rate at whichthe polymers can be quenched. The slow cooling that results can have animpact on the quality of the product film, particularly in the degree ofcrystallinity.

The invention overcomes these difficulties by using superior carriersthat are thinner and that exhibit lower adhesion to the polymeric filmsand thereby better facilitate strippability. These materials alsopossess lower thermal mass, facilitating rapid quenching to yieldexceptionally low crystallinity in the films. The carriers used in theinvention are also less stiff and may be tracked using rollers incontact with the surface of the coated carrier belt, a method allowingvery high speed operation at continuously variable widths. This latterfeature can significantly reduce material yield losses.

The actual choice of carrier for any given film is dictated by thehighest process temperature it will encounter, the work of adhesiondeveloped between the carrier and the film surface in direct contactwith it, and its chemical compatability with the casting fluids. Ingeneral, the carrier should be of low thermal mass, dimensionally stableat the maximum processing temperature, chemically resistant to allcomponents of the casting fluids, and the work of adhesion between thedeposited film and the carrier surface must not exceed the yieldstrength of the deposited film. Once these conditions are satisfied, theactual selection of a carrier for any given film from all carriercandidates is a matter of taking into account its useful life as well asits initial cost for the sake of economy.

Suitable carriers for casting of the invention films include:

a) Films of high melting thermoplastics, such as the thermoplasticpolyimides, (e.g. Upilex® from ICI) polyether-ether ketones (e.g.STABAR® from ICI), polyaryl ketones from Union Carbide, polyphenylenesulfide (e.g. RYTON® from Phillips Corp.), and polyetherimides (e.g.ULTEM® from General Electric Co.). High melting perfluoropolymeric filmsmay themselves be used for casting of the lower melting, partiallyfluorinated copolymers, such as TFB 7100D (a terpolymer of VF₂, TFE, andHFP) from Hoechst.

b) Films of thermosetting plastics, particularly of the high temperaturecapable thermosetting resins such as polyimides (e.g. Kapton® H fromDuPont) are particularly good carriers since they possess excellent hightemperature thermal and dimensional stability as well as durable releasecharacteristics. The surface free energy of the Kapton® H is reported tobe about 45 to 55 ergs/cm, yet has somewhat surprisingly proven to be anexcellent candidate for accepting the casting fluids which typicallyhave a surface tension of about 29 to 35 dynes/cm. It is suspected thatcertain additives in the casting fluids in someway abets wettability.

c) Coated or laminated textiles based upon the above thermoplastics orsimilar thermally stable resins and thermally stable reinforcements suchas fiberglass, graphite, polyaramid (e.g. Kevlar®), and aromaticpolyamide (e.g. Nomex®) yarns may also be used as a carrier to maximizedimensional stability at high temperature as opposed to an unsupportedfilm. To avoid excessive stiffness in an otherwise suitable coated orlaminated textile, it is desirable to employ a more flexible coatingresin as a subsurface coating followed by a top-coat or lamination ofthe otherwise desirable, but too stiff composite. For example a PTFEperfluoroplastic or "Kalrez" perfluoroelastomeric coating on a thin,woven fiberglass substrate (e.g. Style 104 or 116) may be provided witha polyimide surface by top-coating or laminating.

d) Plastic Coated Metal Foil may be used as a carrier. While a thinmetal foil, such as a 3 mil aluminum foil, has the disadvantagespreviously cited, a relatively thin coating of one of the aforementionedthermoplastics or thermosetting resins or a thin metal foil couldprovide an acceptable casting medium essentially without thosedeficiencies.

e) Metallized or Metal Foil Laminated Plastic Films may be used ascarriers. Any of the acceptable plastics, or even elastomers, in thinsheet or film form could be metallized or laminated between very thinmetal foils to provide the good wettability and release properties ofthe metal while eliminating their disadvantages. In particular, a hightemperature cured fluoroelastomer even in very thin (about 2 mil) sheetform sandwiched between thin aluminum foils could have excellent utilityas a casting medium. Similarly the coated or laminated textilesmentioned in (c) above could be laminated between metal foils to providea metal surfaced, dimensionally stable high temperature carrier withexcellent toughness (tear resistance) to improve durability in use,compliance to roll and metering surfaces in the equipment, whileoffering excellent strippability and fluid wettability.

It is clear from this discussion that a large number of carrier optionsexist for the invention process which go well beyond the metal carriersof the prior art. This is true not only for lower temperature processesfor casting low melting polymers, but even for the highest meltingperfluoropolymer (PTFE) for which a polyimide casting surface is in factpreferred, and which exhibits excellent performance as demonstrated inthe Examples. Such a surface is also suitable for casting TFE copolymerswith perfluoro (propyl vinyl ether) such as Teflon® PFA. This latterobservation is rather surprising since the Kapton® F products are basedupon reasonably well-bonded Teflon® FEP and PFA coatings on Kapton® H.This would seem to speak to a requirement for close control of thethermal history of such copolymers in contact with a carrier containingpolyimide resin on its surface as a film or coating.

2. Nature of the Casting Fluids

The casting fluids of the prior art contain about 12% of the surfactantTriton® X-100 (octyl phenoxy polyethoxy ethanol) by weight based uponresin solids and it is taught that this is required to improve thewetting characteristics of the casting fluids on the metal carrier. Itis also taught that 6% Triton® X-100 is insufficient for uniformwetting, while more than 12% leads to non-uniform film thickness byincreasing the viscosity of the fluid.

In contrast, the Examples in accordance with the present inventionindicate that 6% Triton® X-100 is effective in the dispersions employedto produce high quality films at high linear web rates. Thus, it appearsthat the incorporation of a metering device allows a reduction in theamount of surfactant used in the prior art processes. While notintending to be limited to a particular mechanism, it is believed thatthis occurs because the prior art process relied upon the solids level(specific gravity of the casting fluids) to control the thickness of thedeposited resin on each pass. The viscosity of such dispersions is verylow, generally less than about 17 cp at 60% solids, and this lowviscosity is required to limit the buildup of resin to less than about0.37 mils per pass the critical cracking thickness of such dispersions;i.e. the thickness above which mud-cracked deposits form upon drying ofthe dispersion.

Using the invention, however, it was found that excellent quality filmsmay be produced at high carrier speeds, and at more modest surfactantlevels (6% Triton® X-100), and that solids levels up to about 60% may beemployed to obtain films of excellent quality. This is advantageous asit reduces the amount of surfactant and dispersion liquid that must beremoved in the drying zone and the bake/fuse zone. Further, the wettingcharacteristics of the casting fluids may be controlled by means ofadditional fluorosurfactants, for example fluorinated alkyl polyoxyethylene ethanol surfactants such as Fluorad® FC-170C from 3M orsilicone-based surfactants such as Union Carbide's L-77. Thesesurfactants are effective in reducing the surface tension of the castingfluids in much more modest quantities than is Triton® X-100, and theycan be more rapidly eliminated by thermal decomposition, volatilization,or sublimation in the baking zone of the invention art equipment.

It was also surprisingly found that uncracked resin deposits can beformed at thicknesses well in excess of the critical cracking thicknessassociated with the dispersions employed. As mentioned in Example 5hereinbelow, this is not fully understood but may relate to the factthat one of the ionic additives employed, FC-170C, is known to promote arapid increase in fluid viscosity at elevated temperatures in the caseof Triton® X-100 containing dispersions. It is speculated that as thedispersion starts to dry at the very rapidly increasing temperaturescharacteristic of high speed processing, the drying resin has less timeto drain itself of water prior to evaporation, resulting in a thickerresin deposit of lower apparent density prior to complete drying.Consequently, the deposit is less prone to crack if the shrinkage forceswhich induce cracking are dependent upon more intimate particulatecontact. The well known tendency of dispersion--derived PTFE tofibrillate upon intimate particulate contact may in fact be thephenomenon responsible for the mud cracking ordinarily observed whenPTFE dispersions dry in the presence of more modest time/temperaturegradients than those characteristic of the invention process. This couldalso account for the surprisingly good mechanical qualities of the filmsmade by the invention process after final fusion.

It is desirable, in general, to identify the most appropriatehydrocarbon surfactant(s) for any given casting fluid which incombination with relatively minor quantities of fluorosurfactants yieldsthe desired result of high deposition rates (build per pass) withoutcracking, and facile decomposition, volatilization, or sublimation ofnon-polymeric additives. Since the maximum temperature desirable forfilm consolidation upon final fusion will depend upon the melting pointof the specific polymers in the films, the optimum level and chemicalnature of such surfactants can be different for various filmcompositions. Ionic additives other than the fluorosurfactants may beemployed to advantage in casting fluids to engender a rapid increase inviscosity upon drying. These could include salts such as ammoniumacetate, or other salts equally fugitive in the process, or salts suchas potassium chlorate which can induce decolorization of the fused filmsat very minor levels. Such casting fluids which contain specificadditives contributing to the high quality of the films produced by theinvention process are well beyond the simple surfactants associated withthe prior art process for film casting.

3. Nature of the Methods Used to Apply the Casting Fluids to the Carrier

The prior art casting process is essentially a free dipping process inwhich the only significant controlling factors of the amount of resindeposited on the carrier are the solids level in the dispersion andlinear carrier rate. Thus, for any given fluid, the web speed is limitedto a maximum carrier speed above which the pick up of resin on thecarrier exceeds the critical cracking thickness. This limitation,combined with the limitations imposed by metal carrier belts and thedeterioration of product quality noted at high carrier speeds led tousage of carrier speeds of 3 to 8 linear feet per minute in most cases.

In the method of the invention, however, metering bars are used toenable much more rapid linear travel of the carrier, up to at least 6×that of the prior art process. In the process of the invention, thespeed of the carrier belt is limited essentially only by the length ofthe drying/fusing zone, i.e., the carrier cannot move so rapidly thatdrying does not occur within the drying zone provided and fusion withinthe fusing zone provided. The wiping action of the metering bars removesthe excess casting fluid associated with high speed carrier travel sothat an uncracked deposit of dried resin may be obtained prior to finalfusion of that deposit.

The selection of metering bars, however, is not trivial since it isundesirable to introduce shearing of the casting fluids sufficient tocoagulate the resin contained in the reservoir between the cavities ofthe metering bar and the moving carrier. The size and shape of themetering cavity is dependent upon the shear stability of each specificcasting fluid. Additives to minimize polymer shearing by the meteringbars may also be used in the casting formulations. For example, foamingat the metering bars over an extended period of time could introduceunacceptable shearing at the bars. This may be ameliorated by using anantifoam such as Dow-Corning FG-10, as well as fluorosurfactants such as3M's Fluorad® FC-146 (perfluoroammonium octanoate). Since the inventionfluids can contain widely different polymers with particles of varyingshear sensitivity, variable solids content, particle size, andsurfactant systems, the selection of a bar geometry (cavity size andshape) specific to any given casting fluid is more difficult to modelthan to identify by trial and error. The Examples provided hereinindicate the general variety of bars (and therefore cavities) which havefacilitated the production of high quality films by the inventionprocess at high productivity.

4. Nature of the State of Consolidation of the Casting Fluids and ResinsFrom Metering Through Drying and Fusion

The prior art process may be characterized as providing an effective andvery simple means to deposit casting fluids on a carrier at a rate whichis limited by the critical cracking thickness of the castingformulation. The drying of the casting fluids of the prior art processoccurs at a relatively modest thermal gradient and over a relativelylong time and is a function of the web speed and drying/bakingtemperatures. Fusion and recrystallization occurred over periods ofabout 2.5 minutes up to as much as 35 minutes or more, leading to goodquality films, but low productivity.

In the method of the invention, consolidation from resin containingfluid to the final fused film proceeds over a much shorter time.Specifically, the total residence time in each of the drying zone andthe bake-fuse zone is preferably less than about 1.5 minutes, mostpreferably less than 1 minute and may be substantially shorter. This canlead to a higher than critical cracking thickness build rate ofuncracked resin deposits which can increase real productivity by atleast 30% in the case of monolithic PTFE films. Combining thatproductivity with even a 4× improvement in linear travel rate for thecarrier can lead to a 520% improvement in space/time yields, i.e., thenumber of pounds of film produced per hour relative to the prior artprocess in the case of PTFE. Other resin formulations will havequantitatively different improvements in productivity, but would beexpected to be qualitatively similar. This level of improvement in theproductivity of the invention process results, in fact, in space timeyields approaching or exceeding that of a melt extruder forfluoropolymers such as FEP. Thus, the invention process has the desiredcharacteristics of high productivity and high quality at a costcomparable to that of extruded films.

It is characteristic of the invention method that a shorter totalresidence time is employed for evaporation of the water, baking of thedried solids, and fusion of the baked solids to a polymeric melt thathas heretofore been described for fluoropolymer film production. Theactual time during which the polymers are actually in the melt,undergoing consolidation at the highest process temperature, is shorterthan that of the prior art, but nonetheless results in excellentconsolidation as judged by the ultimate tensile strength and elongationof the film produced.

Additionally, the rate of recrystallization/solidification of the meltis much more rapid then in the prior art process. This results in a filmof greater optical clarity, particularly in the case of PTFE containingfilm. Since the rate of recrystallization of PTFE is a strong functionof cooling rate, it is believed that such films produced by theinvention process are either lower in crystallinity level, or thedomains of crystallinity are smaller than can be achieved by prior artmethods of cast film manufacture. This is corroborated by a somewhatlower melting point and heat of recrystallization for PTFE filmsproduced by the invention process.

Thus, films produced by the invention process are expected to havegreater flexural endurance and lower flexural modulus than filmsproduced by the prior art process, particularly in the case of PTFEcontaining films.

The method of the invention can also be used to produce complexmulti-layer films with a very wide range of compositions, some of whichcould not be produced by melt extrusion at all due either to a disparaterang of melting or decomposition temperatures, or to disparate ranges ofmelt viscosities for the resin blends. In particular, the method isuseful for producing fluoropolymeric films in which the fluorpolymer isselected from the group consisting of fluorine-containing homopolymers,copolymers and terpolymers of tetrahaloethylenes, vinyl fluoride,vinylidene fluoride, hexafluoropropylene, perfluoroalkyl vinyl ethers,ethylene and propylene. Moreover, degradation of properties due tothermal exposure is dramatically reduced as a result of the exceedinglybrief exposure to consolidating (fusion) temperatures. Thus, theinvention provides improved productivity and in many cases superiorproducts than the prior art casting process. The invention will befurther illustrated by way of the following nonlimiting examples.

EXAMPLE 1

The casting tower utilized was composed of a dip pan under a 16 footvertical oven divided via air flow into an 8 foot drying zone and abake/fuse zone of 8 feet. Captured within the 8 foot bake/fuse zone wasan electric radiant zone of 4 feet vertical height, with a maximum wattdensity of 22 watts per square inch. After passing through the tower theweb was cooled by a cooling plenum, and then passed over a head roll onits way to the windup. A high quality PTFE film was cast on a 0.005 inchthick polyimide film carrier (Kapton® H from E. I. DuPont) in fivesuccessive passes to obtain a 0.002 inch thick PTFE film with anultimate tensile strength of 6902 psi and an ultimate elongation of695%. To do this, an Algoflon® dispersion (D60 Exp 1 from Ausimont) waslet down to a specific gravity of 1.34 with water. To this was addedUnion Carbide L-77 surfactant at 1.0% by weight of the liquid. Thisdispersion was metered onto the carrier by using 1 inch diameterstainless steel metering bars, wound with 0.040 inch stainless steelwire on each face of the carrier. The metering bars were located about12 inches above the dispersion bath with 2 inches vertical separationand approximately 1/2 inch overlap. The carrier was run at 17 fpm. Thetower heat conditions were determined by the following set points:drying zone/250° F., bake-fuse zone/300° F., radiant electric zonecontrolled to give a web temperature of 770° F. as measured by anoptical pyrometer. Multiple test samples (1/2"×8") gave an averageultimate tensile strength of 6240 psi and an average ultimate elongationof 595%. The quality of this film, produced at a linear rate about 3.4×that of prior art casting methodology, is significantly better than theprior art films produced by casting at lower carrier rates with longerdrying and baking times (4000 to 4500 psi and 400 to 450% ultimatetensile strength and elongation, respectively).

EXAMPLE 2

A multi-pass high quality film was produced on the pilot tower(previously described in Example 1) utilizing the following procedure.Algoflon® D60 Expl resin was let down to 1.49 specific gravity with a 6%Triton® X-100 solution in water. To this was added 200 cc per gallon ofa 5% stock solution of 3M fluorosurfactant FC-170C in water. Also addedat 2%g by weight of solids was a gold pigment (Mearl Corp Super Gold239Z). This dispersion was metered directly onto a 0.005 inch thickpolyimide (Kapton® H) carrier using 1/2 inch diameter #22 wirewoundmetering bars for four passes (bars located as previously described )and a coarser metering bar (1/2 inch diameter bar wound with #30 wireoverlayed with #5 wire) for the fifth pass. The dispersion bath was thenchanged to DuPont T30B PTFE resin dispersion which had been let down to1.40 specific gravity with a 6% Triton® X-100 (Rohm and Haas) solutionand 100 cc per gal. of the previously described 5% fluorosurfactantstock solution. To the dispersion was added 8% by weight of solids ofBorden's Aquablack AB 135 pigment. Two passes of this black formulationwere then metered onto the "Gold" PTFE cast film using the "5 over 30"metering bars. A final pass of DuPont's TE9503 FEP dispersion at a 1.25specific gravity (let down with water) was then metered on as a bondinglayer. All passes were run at a web speed of 22 fpm. The previouslydescribed casting tower had a drying zone temperature set point of 250°F. with a bake and fuse zone temperature set point of 680° F. Theradiant zone was set to control at 760° F. for the gold passes and at720° F. for the black passes and the FEP pass. The final film was 0.0039inches thick, readily removed from the carrier, and of excellentphysical properties for a filled resin. Physical testing indicated anultimate tensile strength of 5244 psi and a 470% ultimate elongation, atleast as good a quality as that which would be obtained by prior artcasting methodology but obtained at a 4.5× web rate.

EXAMPLE 3

To show the effect of reduced thermal mass, a 0.002 inch thick PTFE filmwas cast on a 0.003 inch thick aluminum foil carrier in six passes usingthe previously described casting tower. The PTFE dispersion was DuPont'sT30B let down to a 1.34 specific gravity with water. The dispersion wasmetered on to the aluminum foil using 1 inch diameter, #40 wirewoundmetering bars, as described in previous Examples, at 14 fpm carrierspeed. The tower was set for a drying zone temperature of 250° F. and abake-fuse zone temperature of 680° F. for the first two passes. Theupper zone was increased to 710° F. for the next four passes. The finalfilm was stripped from each side of the carrier with minimal difficulty,and had a tensile strength of approximately 4500 psi, as good as thatobtainable with prior art casting methodology but produced at a 2.8×linear web speed. Although this unmodified aluminum foil carrier isprobably unsuitable for routine use as it annealed during the bake/fusestep, it shows that reduced thermal mass carrier with aluminum surfaceswould be suitable if used in forms less sensitive to thermal effects asdescribed hereinabove.

EXAMPLE 4

A very high quality thin PTFE film was cast in four passes at 25 fpm webspeed for each pass. The previously described casting tower was set fora drying zone temperature of 250° F., a bake-fuse temperature of 680°F., and a radiant zone control temperature of 770° F. The PTFEdispersion employed was Algoflon® D60 Exp 1 which had been let down to1.50 specific gravity with a 6% Triton® X-100 and water solution. Tothis was added the previously described 5% stock solution of FC-170C(100 cc per gallon of dispersion). The dispersion was metered onto the0.005 inch thick polyimide film carrier (Kapton® H) using 1/2 inchdiameter, #14 wirewound bars. The resulting 0.011 inch thick film had anultimate tensile strength as high as 7029 psi and an ultimate elongationof 550%. This dramatic improvement in quality relative to similar filmsproduced by the prior art casting methodology was obtained along with a5× improvement in productivity as measured by relative linear web rates.

EXAMPLE 5

In this example a high quality film was achieved using high speed filmproduction techniques on a 0.005 inch polyimide carrier (Kapton® H)using the previously described casting tower. The PTFE dispersion wasAlgoflon® D50 Exp 1 (Ausimont) let down to 1.50 specific gravity with a6% Triton® X-100 solution and 100 cc per gal. of FC-170C 5% solution aspreviously described. The dispersion was metered onto the carrier in sixpasses using 1/2 inch diameter, #26 wirewound bars at 25 fpm linear webspeed. The final film stripped easily from the carrier and was 0.038 to0.040 inches thick. Its ultimate tensile strength was as high as 6350psi and its ultimate elongation was 640%: a forty percent improvement intensile strength and elongation with a 5× improvement in productivityrelative to prior art casting methodology. The thermal settings for thisrun were: drying zone/250° F., bake-fuse zone/680° F., radiant zone/autocontrolled at 770° F. The build of resin per pass was in excess of0.00067 inches, significantly greater than the 0.0004 to 0.00045 inchesone would predict from measurements of critical cracking thicknessassociated with this dispersion. While not fully understood, thisbehavior is reproducible and is believed to be associated with thedynamics of the fluid consolidation to a dried but uncracked film (whilestill unfused) in the thermal environment of this particular tower. Thiscondition appears to lead to uncracked films, surprisingly about 50%thicker than expected and of very high quality.

EXAMPLE 6

In this example a high quality film was achieved using high speed filmproduction techniques on a 0.005 inch polyimide carrier (Kapton® H)using the previously described casting tower. The PTFE dispersion wasDuPont T30B let down to 1.33 specific gravity with water. The dispersionwas metered onto the carrier using 1 inch diameter, #40 wirewoundmetering bars, yielding a 3.3 mil film in seven passes. The thermalsettings for this run were: drying zone/250° F., bake-fuse zone/710° F.,radiant zone/auto controlled at 780° F. The carrier rate was 26 feet perminute.

The following Table indicates some significant differences between thisfilm, cast PTFE films of the prior art, and skived PTFE film.

    __________________________________________________________________________                  Tensile Strength                                                                       Ult. Elongation                                                                       Elastic Modulus                                        Thickness                                                                           MD  TD   MD  TD  MD  TD        Hm                                       (mils)                                                                              (psi)    (%)     (×10.sup.-3 psi)                                                                Tm °(C.)                                                                     (J/g)                            __________________________________________________________________________    Skived PTFE                                                                   (Chemplast)                                                                           1.9   7580                                                                              5860 450 360 68  75  327.6 ± 0.1                                                                      21.2                             Cast PTFE                                                                             1.7   4580                                                                              5250 420 520 67  64  325.0 ± 0.1                                                                      22.8                             (Prior Art:                                                                   Toralon DF-100                                                                Cast PTFE                                                                             3.3   5190                                                                              5000 530 510 63  63  324.2 ± 0.1                                                                      19.1                             (Invention Art)                                                               __________________________________________________________________________     MD = Machine Direction, TD = Transmission Direction                      

These data demonstrate the more anisotropic elongation of the cast filmsvs skived films, with the invention film being clearly the mostisotropic. The reduced elastic modulus of the cast films is alsoevident.

The melting pint (Tm) of the cast film made by the invention method issignificantly lower than the prior art cast film and its heat of fusion(Hm) is drastically reduced. The observation of pure PTFE with a meltingtemperature below 325° C. and a heat of fusion less than 22 joules/gramis indicative of a film with greater optical clarity suggesting lowercrystallinity or smaller crystallinites. Further, this behavior shouldextend to mixtures of PTFE with other materials, although the exactnumbers will depend on the proportions and the amount of effect onfreezing point due to interactions of the polymers.

We claim:
 1. A fluoropolymeric film comprising at least onefluoropolymer-containing layer formed by(a) preparing an aqueousdispersion comprising a fluoropolymer; (b) dipping a carrier beltthrough the dispersion such that a coating of the dispersion is formedon the carrier belt; (c) passing the coated carrier belt through ametering device to remove excess dispersion; (d) drying the meteredcoated carrier belt to remove the water from the dispersion; (e) heatingthe dried coated carrier belt to a temperature sufficient to consolidatethe dispersion; (f) cooling the consolidated coated carrier belt; and(g) stripping the consolidated fluoropolymeric film from the carrierbelt, wherein the carrier belt is formed from a material of low thermalmass having chemical and dimensional stability at the consolidiationtemperature of the dispersion and a work of adhesion that does notexceed the yield strength of the consolidated fluoroplymeric film.
 2. Afluoropolymeric film according to claim 1, wherein the fluoropolymer isselected from the group consisting of fluorine-containing homopolymers,copolymers and terpolymers of tetrahaloethylenes, vinyl fluoride,vinylidene fluoride, hexafluoropropylene, perfluoroalkyl vinyl ethers,ethylene and propylene.
 3. A fluoropolymeric film according to claim 1,wherein the film comprises a plurality of fluoropolymer-containinglayers formed by successively repeating steps (a) through (f) and thenstripping the consolidated multilayer film from the carrier belt.
 4. Amethod for preparation of a fluoropolymeric film on a carier,comprising:(a) preparing an aqueous dispersion comprising afluoropolymer; (b) dipping a carrier belt through the dispersion suchthat a coating of the dispersion is formed on the carrier belt; (c)passing the coated carrier belt through a metering device to removeexcess dispersion; (d) drying the metered coated carrier to remove thewater from the dispersion; and (e) heating the dried coated carrier to atemperature sufficient to consolidate the dispersion, wherein thecarrier belt is formed from a material of low thermal mass havingchemical and dimensional stability at the consolidation temperature ofthe dispersion and a work of adhesion that does not exceed the yieldstrength of the consolidated fluoropolymeric film.
 5. A method accordingto claim 4, wherein the carrier belt is dipped in the dispersion at arate of at least 10 linear feet per minute.
 6. A method according toclaim 5, wherein the heating step takes less than 1.5 minutes.
 7. Amethod according to claim 4, wherein the aqueous dispersion furthercomprises a surfactant in an amount effective to facilitate wetting ofthe carrier belt by the aqueous dispersion.
 8. A method according toclaim 7, wherein the surfactant is selected such that it is thermally oroxidatively fugitive at a temperature at or below the consolidationtemperature of the dispersion.
 9. A method according to claim 7, whereinthe surfactant is octylphenoxy polyethoxy ethanol at a concentration ofabout 6%.
 10. A method according to claim 7, wherein the surfactant isoctylphenoxy polyethoxy ethanol at a concentration of about 6% togetherwith a fluorinated ionic surfactant.
 11. A method according to claim 4,wherein the carrier is selected from among films of high meltingthermoplastics, films of thermosetting plastics, coated or laminatedtextiles formed from a thermally stable plastic or resin and a thermallystable reinforcement, a plastic coated metal foil and metallized ormetal foil laminated thermally stable plastic or resins.
 12. A methodaccording to claim 4, wherein the fluoropolymer is selected from thegroup consisting of fluorine-containing homopolymers, copolymers andterpolymers of tetrahaloethylenes, vinyl fluoride, vinylidene fluoride,hexafluoropropylene, perfluoroalkyl vinyl ethers, ethylene andpropylene.